1. Field of the Invention
The present invention relates to adsorbent coatings, and more particularly to adsorbent coatings that are applied to metallic surfaces and a process for applying the same.
2. Description of the Prior Art
Adsorbent beds are substances whose surface or capillary hollow space structure can adsorb a lighter volatile operating medium. Thus the adsorbent beds are typically well suited for heat transformation. European Patent Nos. EP 0 151 237 and EP 0 205 167 disclose typical embodiments of adsorbent beds. These adsorbent beds generally adsorb a vaporous operating medium stored in an evaporator by releasing the heat carried by the operating medium. As is known, the continued vaporization of operating medium generates cold in the evaporator. As a result, operating medium vapor will continue to be removed from the evaporator at lower temperatures. In order to expel the operating medium from adsorbent bed, heat is provided to the adsorbent bed which serves to again convert the operating medium to a vapor which is then reliquified in a condenser. In order to achieve rapid adsorption and expulsion of the operating medium by the adsorbent bed, the adsorbent bed should have a good heat conductivity in the adsorbent bed and a good heat transfer to heat exchanger faces.
European Patent No. EP 0 151 786 discloses an adsorption bed blank with a high heat conductivity and a method for making the same. The patent discloses admixing a powder-like adsorption medium (zeolite) with a binder agent and water. The mixture, which is in a semi-liquid state, is poured into receptacles, specifically heat exchangers. After the mixture is dried and gelled, the zeolite adsorption medium is prepared to adsorb operating medium. The zeolite mixture is not applied to strategically mounted flow conduits so as to ensure that an inflow of vapor is not substantially disrupted. One disadvantage of this configuration is that during frequent and rapid temperature changes, the zeolite blank will separate from the heat exchanger surface. Therefore, a rapid heat exchange between the heat exchanger and the zeolite is no longer assured.
A further disadvantage of the above-identified adsorption bed blanks is that structures which contain zeolite generally conduct heat relatively poorly. Despite having good heat exchanger geometries, the time period required for the adsorption and expulsion of operating medium vapor is in the range of from a few minutes to hours. These adsorption blank structures are unsuitable in devices that have strict weight limits such as those that are intended to be mobile.
Commercial zeolites are available as either a powder or granulate, in spherical or cylinder form. Zeolite powder is incapable of being used in the aforementioned adsorption system because the inflow and outflow of operating medium vapor will remove the zeolite powder from the adsorption system. Granulate zeolite may be utilized in the above-described system but, the device still has the aforementioned disadvantages of poor heat conductivity and poor heat contacting. Both natural and synthetic zeolites adsorb a certain amount of operating medium vapor and thereafter again emit the operating medium when heat is applied. The absorbability of zeolites may differ by more than 25% by weight depending upon the type of zeolite. However, for most zeolites, the desorption temperatures for complete expulsion of operating medium must be between 200.degree. and 300.degree. C. But, when the zeolite adsorbs operating medium vapor, the temperature of the adsorption bed blank should preferably be below 100.degree. C. This provides very high temperature differences between the operating medium vapor and the adsorption bed blank which can only be realized with large heat exchanger faces having thin zeolite layers.
Since the desorption temperatures of zeolite are typically above 200.degree. C., the employment of metallic heat exchangers is advantageous. However, due to the differences in heat expansion coefficients between the adsorbent bed coating and the metallic heat exchangers, the adsorbent bed coating typically separates from the heat exchanger. When cracks develop between the adsorbent bed coating and the heat exchanger faces, heat transfer between the adsorbent bed coating and heat exchanger is severely limited. This results in longer cycle times and requires larger amounts of adsorbent bed coatings for the given intended application. In addition to a good contact between the adsorbent bed coating and the metallic heat exchanger, a sufficient input of operating medium vapor must be assured for proper operation. However, an adsorbent bed coating that maintains relatively good heat contact with metallic surfaces such that a sufficient input of operating medium vapor is assured and the adsorbability of the adsorbent bed is uninterrupted is not previously known.